1. Field of the Invention
The invention relates to a method of fabricating an integrated circuit, and more particularly relates to a method of fabricating a flash memory that has improved conformal oxide covering gates.
2. Description of the Related Art
The split-gate flash memory has some advantages over the stack gate flash memory such as lower program current and the over-erase free issue. However, the process steps for manufacturing the split-gate flash memory are more complicated and the most important problem is the polysilicon stringers that result after a selective gate etching process. The polysilicon stringers induce read disturb problem in the split-gate flash memory.
A method of fabricating the split-gate flash memory comprises the steps of forming a gate on a substrate, then forming silicon nitride spacers on the sidewalls of the gate, and so on. The silicon nitride spacers on the sidewalls of the gate are important for program efficiency and data retention. The processes typically comprise a step of forming a peripheral gate oxide before the silicon nitride spacers are formed. After forming the silicon nitride spacers, the substrate must be dipped in HF solution to remove the remaining oxide exposed by the silicon nitride spacers on the sidewalls of the gate before a gate oxide is formed on a peripheral region.
FIG. 1 is schematic, cross-sectional view illustrating the substrate after dipping in the HF solution according to above-mentioned method. Referring to FIG. 1, reference numbers 10, 12, 14, 16 respectively indicate the substrate, the gate, the oxide layer and the silicon nitride spacers. The oxide layer 14 under the silicon nitride spacers 16 suffers an undercut, indicated by are 18. This undercut phenomenon causes the polysilicon stringer problem and the read disturb problem.